Friday, January 25, 2013

Save them with Poison

By Mark Hamer

Something bizarre repeatable struck my eye as I spent time
hiking through Southwestern Australia in December 2012. It wasn’t the massive Eucalyptus diversicolor, or the cacophony
of bush bird’s singing in their canopies – though these were quite striking - but
rather the ill-fitting signs warning of a pristine landscape poisoned with
something called “1080”. Signs, much like the one pictured below, were boldly
displayed at the entrance to almost every trailhead, park entrance, or
secondary road I came across. It became clear that whatever this poison was,
the Australians didn’t seem at all afraid to use it; so much so that it could
be found in even the most protected of wildlife sanctuaries.

A typical sign warning that 1080 baits have been laid throughout a given area.

It turns out “1080”, better known to nobody except the most chemistry-inclined
as sodium 2-fluoroacetate, is actually a fairly common poison to the continent
of Australia. In fact, it is the poison’s ubiquity that lends it so useful, though
we’ll get to that shortly.

Physiologically,
1080 acts as a metabolic poison and is most toxic when ingested orally by
mammals. Once consumed, the toxic effects can take place over hours or even
days. Sodium 2-fluoroacetate’s toxicity lies in its similarities to a key
molecule in cellular energy metabolism: acetate. Under this mechanism, soluble
flouracetate associates with coenzyme A to form fluoroacetyl CoA. Fluoroacetyl
CoA then enters the TCA (tri-carboxylic acid) cycle where it reacts with
citrate synthase to produce fluorocitrate, a metabolite of which then binds
very tightly to aconitase. This effectively halts the TCA cycle, leading to catastrophic cellular starvation and eventual necrosis. Sublethal doses may result in tissue damage, especially to tissues with high energy demand. Urinary excretion usually takes place within 72 hours of ingestion.

A schematic covering the major steps of the citric acid cycle.

At this point you may be asking: why are Australians putting
1080 poison everywhere and what the hell are they thinking? To my surprise, I
learned that this was not an isolated exercise. Much of Australia, Tasmania, and
New Zealand utilize 1080 poisoning. As it turns out, 1080’s
effectiveness against mammals makes it an amazing tool, which has been exploited
to deal with the issue of rampant invasive species proliferation. Its usefulness is reinforced by the fact that, though it is inherently toxic to most non-microbial species, the LD50’s (which is defined as
the lethal dose at which 50% of animals die) in other animal classes like Amphibia and
Aves are relatively high. The relative insusceptibility of amphibians to 1080
is especially informative, as it is usually these species that are severely
affected by toxins and pollutants that leech in to the environment.

Synthetically manufactured into pellets (see picture to right), sodium
2-fluoroacetate is also found naturally in approximately 35 native Australian
plants. It is water soluble and degradable by many of Australia's native soil bacteria. Australian mammals, most of which are marsupials, have been
allowed to evolve in the presence of this poison for millennia. This has lead
to many native species with the ability to avoid, and in many cases even
tolerate, an otherwise deadly poison. Meanwhile, species like red fox (Vulpes
vulpes), feral cats (Felis catus), and European rabbits (Oryctolagus
cuniculus) show extreme sensitivity to 1080 with LD50 of 0.12mg/kg, 0.4mg/kg,
and 0.37mg/kg respectively. Species that show active hunting behaviors like the fox are thus easily targeted with baits containing the poison. Sadly, domesticated dogs are one of the most susceptible animals to this poison with a LD50 of about 0.07mg/kg.

Red fox, feral cats, and European rabbits all pose a very real threat to native Australian species as they outcompete them for food, water, and territorial resources.

The use of 1080 poison in Australia has been met with reasonable success, but there is fear that its effectiveness may not last. European rabbits have been shown in studies to adapt a tolerance to the poison. This is especially worrisome for r-selected animals like the rabbit. Tolerance in these animals could rapidly grow to the point where 1080 becomes an ineffective method of control. K-slected populations, like foxes, won't likely develop such tolerances in the foreseeable future.

Surprisingly, very little is actually known about the physiological modifications that some australian mammals have acquired to desensitize them to the effects of 1080. What is known is that the

rate of de-fluorination does not play a significant role in this acquired tolerance. A study in the 1960's using fruit flies (Drosophila melanogaster) showed acquired resistance to 1080 after only 67 generations. Frustratingly, it seems there is little interest in the mechanism of mammalian resistance. It is likely, however, that modifications to certain proteins (perhaps aconitase?) involved in the TCA cycle are responsible.

References:

Gilbert S. 2012. A Small Dose of Toxicology: The Health
Effects of Common Chemicals. Print.

Mcilroy, J.C.
1986. The Sensitivity of Australian Animals to 1080 Poison: Comparisons Between
the Major Groups of Animals, and the Potential Danger Nontarget Species Face
From 1080 Poisoning Campaigns. Australian
Wildlife Research 13: 39–48.

Mcilroy, J.C., D.R.
King, and A.J. Oliver. 1985. The Sensitivity of Australian Animals to 1080
Poison VIII.* Amphibians and Reptiles. Australian
Wildlife Research 12: 113–118.